A variable frequency drive (VFD) is an electronic device that controls the speed of AC induction motors. Before we look at how this works and how it can be used, we should look at the history of motor controllers, and also how the induction motor itself works.
It has always been useful to control the speed of electric motors used in industry. Nearly every process that uses a motor will benefit from speed control. Not only is the process generally improved, but in many cases (particularly with pumps and fans) these is considerable energy saving.
Before electronic controllers were available, motors were controlled in various ways, for example by controlling the field current on a DC motor using a series of resistors, or by using other motors. However, when Thyristors, the first power semiconductors, became available in the 1950’s, it became possible to control the armature voltage, and therefore the speed, of a DC motor using phase control. These DC drives are still manufactured and in wide use today.
However, the challenge has always been to control the speed of the AC induction motor, also known as the Asynchronous motor. While a DC machine usually has two wound parts (the field and armature windings) as well as brushes and a commutator, the AC machine has a simple, fixed winding (the stator) and a rotor. The rotor usually consists of conductors formed by casting Aluminium or Copper in the iron core. There are no brushes or commutators. The machine is therefore cheaper, simpler and more reliable. It is little surprise that these machines make up the majority of motors used in the world’s industries. So how does it work, and why does it need a variable frequency drive?
Let’s start by looking at a three phase transformer, shown in Figure 1
If the transformer winding is connected to a three phase AC supply, a sinusoidal current will flow in the primary windings. The current will cause a magnetic flux to be induced in the iron core of the transformer, which will rise and fall as the applied voltage (and therefore current) alternates, usually at 50 or 60 Hz, depending on the power system.
The changing magnetic flux will then induce a voltage in the secondary windings, and if a load is connected (or even if the windings are shorted together) a current will flow. The ratio of the turns of the primary and secondary windings will determine the ratio of the primary voltage and the secondary voltage, which is why transformers are so useful.
Now imagine that we roll up the windings, make a small air gap between them, and allow the secondary winding, now called the rotor, to move freely. This is the basis of the induction motor, shown in Figure 2.
So when we connect a three phase supply to the primary winding – now called the stator, we have transformer action as before, and current flows in the rotor (secondary) windings. As mentioned above, the rotor usually consists of cast conductors within an iron core with a shorting ring at each end. As this arrangement appears a little like a circular cage (ignoring the Iron of course!) the motor is sometimes referred to as a squirrel cage motor. In the Figure 2 the conductors are perpendicular to the diagram, and the shorting rings not shown.
Figure 3 shows a typical motor construction.
Now if we have a magnetic field and an electric current, we get a force, and according to Fleming’s Left Hand Rule this will turn the rotor, so we have a motor. However, as the motor speeds up, it begins to ‘catch up’ with the magnetic field, which is effectively rotating - alternating - around the stator at the frequency of the three phase supply. Now we will only get transformer action as long as the magnetic field is continually changing; transformers only work with AC. So if the rotor catches up with the supply, there is no longer a changing magnetic field, so there is no transformer action, no rotor current, and no torque. So a standard induction motor will always run slightly slower than the applied frequency. This reduction in speed is known as the slip. As an induction motor is loaded, the slip increases a little, more current is drawn, and the motor takes up the load. So the speed of the motor is basically dependent on the applied frequency. In a simple induction motor, this speed will typically be a few percent lower than the synchronous speed (at which no torque is available).
Doubling (or trebling etc.) the number of windings or pole pairs allows this speed to be reduced. So a motor with one pole pair (a two pole motor) running on a 50 (60) Hz supply will operate at say 48 (58) revolutions per second, or 2880 (3480) rpm. A four pole machine, the most common, therefore runs at 1440 (1740) rpm. Six and eight pole motors are readily available, with specialist applications requiring more poles using special motors.
Figure 4 shows the classical torque/speed relationship of an AC induction motor.
So if we want to control the speed of the motor we need to vary the applied frequency. However, if we manage to vary the frequency, we need to look after the voltage as well, because the magnetising current in the stator depends on the integral of the voltage over time. That is, the area under the curve of the sine wave. If we decrease the frequency then the period, or length of the sine wave increases, so the area under it also increases, leading to excessive magnetising current in the motor. So if we reduce the frequency we must also reduce the voltage applied to the motor in proportion.
We’ll discuss how we do that electronically with a variable frequency drive in the next article.